It's an adult little blue heron. All herons can pull their necks in like that, which can be confusing.
Edit: First year little blue herons are actually white. Given their size, they are often confused with Snowy Egrets and white-morph Reddish Egrets (the bills and feet are used to tell them apart, but even then it's sometimes very difficult). They are patchy white and blue in the spring the next year, and are fully blue by that summer.